Vat dyeing of acrylonitrile polymers using potassium ions



Patented May 27, 1952 VAT DYEING F ACRYLONITRILE POLY- MERS USINGPOTASSIUM IONS Esther Meryl Griflith, Bloomington, Ill., assignor to E.I. du Pont de Nemours & Company, Wilmington, Del., a corporation ofDelaware No Drawing. Application March 3, 1949, Serial No. 79,521

13 Claims. 1

This invention relates to the modification of polymers of acrylonitrile.More particularly, it relates to the dyeing of structures comprisingacrylonitrile polymers.

This is a continuation in part of m Serial No. 771,704, now abandoned.

By structures is meant shaped articles, such as yarns, films, bristles,fabrics, tubings, molded articles and the like, and by acrylonitrilepolymers is meant those polymers containing at least 85% by weight ofacrylonitrile.

Acrylonitrile polymers containing a major portion of acrylonitrile arerelatively insoluble, unreactive and hydrophobic materials. Thesecharacteristics make the dyeing of structures prepared from suchpolymers a diflicult problem. Indeed, standard commercial dyeingtechniques cannot be used satisfactorily. For example, only light shadesare obtained on dyeing structures of polyacrylonitrile with dispersedacetate, basic and certain vat colors. This is true for copolymers ofacrylonitrile prepared using up to of other vinyl type monomers. Thesecopolymers retain the desirable properties of polyacrylonitrile and showonly a slightly enhanced dye receptivity. Since the physical andchemical properties of shaped articles prepared from acrylonitrilepolymers make the articles of considerable commercial interest,solutions to the difiicult dyeing problems are highly desirable.

Accordingly, it is an object of this invention to provide a satisfactoryprocess for dyeing acrylonitrile polymers. A further object is theprovision of methods for uniformly dyeing structures of acrylonitrilepolymers. A still further object is the provision of dyed structures ofsuch polymers which are deeply and uniformly dyed and which are fast tolight and to washing. Other objects will appear hereinafter.

The objects of this invention are accomplished by treating thestructures of this invention with an aqueous dye bath containing a vatdyestufi of the indigo, thioindigo or related type, alkali, a readilyionizable potassium compound and a reducing agent and then oxidizing theleuco dyestuif on the article.

The invention will be more clearly understood by referring to theexamples and discussion which follow. These examples are given forillustrative purposes and are not to be construed in any sense aslimitative. Parts, proportions and percentages are by weight unlessotherwise indicated.

Emample I A dye bath was prepared using 400 parts of Water, 2 parts of6,6'-dibromo-4,4'-dimethyly application 2,2-bis-thionaphthen-indigo, 10parts of sodium hydrosulfite, 0.8 part of sodium hydroxide, and 27.6parts of potassium carbonate (4 equivalent weights per 100 parts ofpolymer). pH of this bath was 11.4. Ten parts of polyacrylonitrile yarnwas kept immersed in the bath for one hour, the bath being heated to 97to 98 C. After this time the yarn was rinsed in cold water and immersedin a bath containing 400 parts of water and four parts of nitric acid.This oxidation bath was heated to 98 C. and the treatment lasted for onehour. After scouring the sample in 0.5% soap solution at 98 C., rinsing,and drying, the yarn was found to be dyed a strong pink shade.

If, in the dye bath above, the potassium carbonate is replaced by fourequivalent weights per 100 parts of polymer of sodium carbonate (21.2parts), only a weak pink shade is obtained on the'fiber after dyeing.The weak shade is similar to that obtained using a dye bath having a pHof 11.2 and containing no salt.

Example II Ten parts of polyacrylonitrile yarn was immersed in a dyebath containing 400 parts of water, 2 parts of5,5-dichloro-6,6'-dimethyl- 2,2'-bis hydrosulfite, 0.8 part of sodiumhydroxide and 27.6 parts of potassium carbonate (4 equivalent weightsper 100 parts of polymer). The pH of this bath was 11.4. The bath washeated to 98 C. for one hour. After rinsing, oxidizing and soaping, asdescribed in the previous example, the yarn was a strong red-violetshade.

If the potassium carbonate in this bath is replaced by sodium carbonate,only a weak shade is obtained.

Example III Ten parts of knit tubing prepared from a copolymercontaining acrylonitrile and 5% styrene was immersed in a dye bathcontaining 400 parts of water, 2 parts of 5,5'-dichloro-7,7'-dibromoindigo, 20 parts of sodium formaldehyde sulfoxylate, 1.5 partssodium hydroxide and 39.2 parts of potassium acetate (4 equivalentweights per parts of polymer). The pH of thisbath was 11.2. The bath washeated to 98 C. and the dyeing continued for one hour. After rinsing,oxidizing and sc aping, as described in Example I, the yarn was a strongblue shade. If four equivalent weights per 100-parts of .polymer ofsodium acetate (32.8 parts) is used in place of the potassium acetate inthis example, a weak shade results, similar to that obtained when nosalt is used.

-thionaphthen-indigo, 10 parts of sodium Example IV Ten parts of a filmprepared from the 90/10 copolymer of acrylonitrile/methacrylic acid wasimmersed in a dye bath containing 400 parts of water, 2 parts of5,5,7,7'-tetrabromoindigo, 20 parts of sodium formaldehyde sulfoxylate,1.5 parts of sodium hydroxide and 25.8 parts of potassium chloride (4equivalent weights per 100 parts of polymer). The pH of thisbath was10.9. The bath was heated to 98 C. and the dyeing continued for onehour. After rinsing, oxidizing and soaping, as described in Example I,the yarn was a strong blue shade. If four equivalent weights per 100parts of polymer of sodium chloride (23.4 parts) is used in place of thepotassium chloride in this example, a weak shade results, similar tothat obtained when no salt is used.

Example V Ten parts of knit tubing prepared from a blend of 95%polyacrylonitrile and 5% polyvinylpyridine was immersed in a dye bathcontaining 400 parts of water; 2 parts of 2thionaphthen2-acenaphthylene-indigo, 20 parts of sodium formaldehyde sulfoxylate and1.5 parts of sodium hydroxide and 34.8 parts of potassium sulfate (4equivalent weights per 100 parts of polymer). The pH of this bath was10.8. The bath was heated to 98 C. and the dyeing continued for onehour. After rinsing, oxidizing and soaping, as described in Example I,the yarn was a strong scarlet shade. If four equivalent weights per 100parts of polymer of sodium sulfate (28. parts) is used in place of thepotassium sulfate in this example, a weak shade results, similar to thatobtained when no salt is used.

Example VI If four equivalent weights per 100 parts of polymer of sodiumcarbonate (21.2 parts) is used in place of the potassium carbonate inthis example, a weak shade results, similar to that obtained when nosalt is used.

Example VII Ten parts of polyacrylonitrile knit tubing was immersed in adye bath containing 400 parts of water, 2 parts of2-(5,7-dibromoindole)-5'-bromo-2'-thionaphthen-indigo, parts of sodiumhydrosulfite and 2 parts of potassium hydroxide. The pH of this bath was12.5. The bath was heated to 98 C. and the dyeing continued for onehour. After rinsing, oxidizing and soaping,

as described previously, the yarn was dyed to an I intermediate shade ofviolet.

Ezvample VIII Ten parts of polyacrylonitrile knit tubing was immersed ina dye bath containing 400 parts of water, 2 parts of6,6'-dibromo-4,4'-dimethyl- 2,2-bis-thionaphthen-indigo, 20 parts ofsodium formaldehyde sulfoxylate, 0.8 part of sodium hydroxide and 157parts of' potassium acetate (16 equivalent weights per parts ofpolymer). The pH of this bath was 11.2. The bath was heated to 98 C. andthe dyeing continued for one hour. After rinsing, oxidizing and soaping,as described in Example I, the yarn was a strong pink shade. If 16equivalent weights per 100 parts of polymer of sodium acetate (131parts) is used in place of the potassium acetate in this example, a weakshade results, similar to that obtained when no salt is used.

Example IX Ten parts of polyacrylonitrile knit tubing was immersed in adye bath containing 400 parts of water, 2 parts of6,6-dibromo-4,4-dimethyl- 2,2'rbis-thionaphthen-indigo, 20 parts ofsodium formaldehyde sulfoxylate, 0.8 part of sodium hydroxide and 314parts of potassium acetate (32 equivalent weights per 100 parts ofpolymer). The pH of this bath was 11.3. The bath was heated to 98 C. andthe dyeing continued for one hour. After rinsing, oxidizing and soaping,as described in Example I, the yarn was an intermediate pink shade. If32 equivalent weights per 100 parts of polymer of sodium acetate (262parts) is used in place of the potassium acetate in this example, a,weak shaderesults, similar to that obtained when no salt is used.

Example X Example XI Woven fabric (250 parts) prepared from high- 1ydrawn (9X) polyacrylonitrile yarn was dyed under pressure in anautoclave using an aqueous dyebath prepared according to the followingformula:

5000 parts water 12.5 parts6,6-dibromo-4,4-dimethy1-2,2-bisthionaphthen-indigo 25 parts causticsoda 25 parts sodium hydrosulfite 2.5 parts high molecular weightalkanol sulfate (wetting agent) 25 parts sodium formaldehyde sulfoxylate500 parts potassium carbonate (2.9 equivalent weights per 100 parts ofpolymer) The dye paste was vatted at 82 C. with half the water and halfthe caustic soda and sodium hydrosulfite required in the above formula.The remaining components of the formula were then added.

The fabric was placed in an autoclave on a revolving reel. The dyeliquor was added and the autoclave sealed. The temperature of 127 C. wasmaintained for 15 minutes by holding the pressure at 20 p. s. i. gauge.After cooling the autoclave, the fabric was removed, rinsed well inWater, and oxidized for 15 minutes in a boiling aqueous solutioncontaining 5 grams/liter acetic acid and 5 grams/liter of 30% H202.fabric was then rinsed in water and secured for 15 minutes in boilingaqueous solution containing 5 grams/liter'of soap. Magnified cross-sewThe dyed tion'photographs of the dyed fibers showed that penetration bythe dyestuff was complete. The dyeing was level and the fabric did notcrock under severe test. The physical properties of thepolyacrylonitrile fibers were not impaired by this pressure dyeingtechnique.

The pressure pad-steam vat dyeing technique described by Scull in U. S.2,424,857 was tried at 127 Crwith little success. Color was applied tothe polyacrylonitrile fabric but the dyed piece was most unlevel,crooked slightly, and the fibers were found upon microscopic examinationto be dyed merely onthe surface.

Vat dyeing must be done in alkaline media because the reduced leuco formof the dyestuff is insoluble in acid media. When acrylonitrile polymersare vat dyed in baths having a pH below but above 7, weak shades result.The addition of ionizable potassium compounds to such baths does notimprove the dyeing appreciably. Accordingly, in the process of thisinvention enough alkali should be present in the bath to maintain a pHof 10 or more throughout the dyeing. However, when the pH exceeds 12.5,the enhanced dyeing effect afforded by the presence of potassium iondiminishes rapidly. Therefore, it is preferred to operate the dyebathsof this invention at a pH range of 10 to 12.5. These highly alkalinebaths exert no adverse effect on the acrylonitrile polymers of thisinvention.

Any alkali capable of giving the desired pH may be used in thisinvention. For example, lithium, sodium, and potassium hydroxides may beused. As disclosed in Example VII, potassium hydroxide may be the solesource of potassium ions. In such instances, however. only intermediateshades are obtained since the maximum optimum pH value of 12.5 isreached before a sufficient potassium ion concentration is reached. Forthis reason, it is preferred to use other potassium salts along withpotassium hydroxide. It is also possible, as shown in Example X, to usepotassium salts of weak acids, such as potassium carbonate, as the onlyalkali and as the source of potassium ion.

Further, other inorganic or organic alkaline materials may be used inthe dye bath. For example, as disclosed in copending application SerialNumber 771,379, new U. S. Patent No. 2,543,994, aromatic amines may beused effectively as carriers in the vat dyeing of acrylonitrilepolymers. The amines disclosed in that application may be added to thebaths of this invention. Likewise, potassium salts of such amines asanthranilic and sulfanilic acid may be added.

In general, potassium ion from nearly any source can be used in theprocess of this invention. In addition to the potassium compoundsdisclosed above, any of the following compounds, among others, may beused, either alone or in mixtures: potassium bromide, potassium iodide,potassium fluoride, potassium arsenate or the arsenite, potassiumbicarbonate, potassium cyanlde, potassium sulfide, potassium nitrite,potassium nitrate, potassium phosphate, potassium sulfite, potassiumthiosulfate, potassium hydrosulfite, and potassium salts of organicacids, such as potassium succinate, oxalate, tartrate, formate,propionate or benzoate. Thus salts of inorganic and organic acids whichare ionizable in the dye baths are operative in this invention. It ispreferred to use potassium compounds which do not cause oxidation underthe conditions of dyeing employed. In general, any hot-water solublepotassium compound may be used. It

iii

may be added to the dye bath either before or after the dyestuff isreduced to its leuco form.

The amount of potassium ion used in the practice of this invention canbe varied over wide limits. It is preferred to use between 4 and 20equivalent weights of the potassium compound per 100 parts of polymer toget strong shades. Intermediate shades, which are satisfactory for somepurposes, can be obtained with concentrations as low as two equivalentweights or as high as 32 equivalent weights per 100 parts of sample. Theacrylonitrile polymers which are generally used commercially for thepreparation of the shaped articles described in this invention must, ofcourse, be of a sufficiently high molecular weight to possess fllmorfilament-forming properties. The polymers employed possess an averagemolecular weight within the range of 25.000 to 750,000 or even higherand preferably between the range of 40 ,000 to 250,000 as calculatedfrom viscosity measurements by the Staudinger equation:

N Molecular we1ght-- wherein:

K,,,=1.5X l0 viscosity of solution N", specific VlSGOSlty andC=concentration of the solution expressed as the number of moles of themonomer (calculated) per liter of solution.

It is to be understood, however, that acrylonitrile polymers havingmolecular weights below or above the range indicated may be subjectedsuccessfully to the process of this invention.

The acrylonitrile polymers of commercial interest are those containingby weight of the polymer of acrylonitrile. These polymers includepolyacrylonitrile and copolymers and interpolymers of acrylonitrile withother polymerizable monomers. These monomers include, among others,vinyl acetate, vinyl chloride, acrylic and methacrylic acids and theirderivatives or homologues, methyl vinyl ketone, vinyl pyridine andhomologues thereof, and isobutylene or other polymerizable hydrocarbons.Polymers containing less than 85% of acrylonitrile can usually be dyedby standard processes, since inclusion of greater amounts of such materials as acrylic acid, methyl vinyl ketone and the like, leads tobetter dye receptivity. If desired, the process of this invention can beused to dye such polymers. However, since it is preferred to usepolymers having at least 85% acry-' lonitrile to get the benefit ofsuperior properties, such as toughness and chemical inertness, theprocess of this invention is chiefly of interest in the dyeing ofstructures prepared from polymers containing a major portion ofacrylonitrile.

The yarns described herein are prepared by conventional wet or dryspinning techniques from solutions of polyacrylonitrile or copolymerscontaining 85% or more acrylonitrile in dimethyl formamide,tetramethylene sulfone or other known volatile solvents for thesepolymers as described in United States Patents Nos. 2,404,714 to2,404,727, inclusive. On leaving the spinning cell, the yarn iscollected in a suitable package and this yarn package may then be washedfree of residual solvent. In order to obtain full bene fit of theproperties of these yarns, it is desirable to draw the yarn from" 2 to10 times its original length. Moreover, when heated under tension forextendedperiods of time,-the yarns show a remarkable retention oftenacity; while, on the other hand, if the yarns afterdrawing are heatedin the-relaxed state at temperatures in the order of 130 to 200 C'., theyarns tend to shrink somewhatand such an after-treatment can be used toincrease the elongationof the yarns to apoint where they aresatisfactory for use in the textile art.-

The structures such as yarns, films, fabrics and the like maybe preparedbythe usual techniques, for example, yarnsor fibers may be prepared asdescribed above by dry spinning or wet spinning according to suchprocedures described in ,copending applications Serial Numbers 496,376,735,666, and 746,651, now U. S. Patents Nos.'2,426,719; 2,451,420;and2,467,553, respectively. Films may be prepared in similar ways, or bycasting techniques. Fabrics of the new compositions of matter of thisinvention may be prepared by any of the well-known knitting or weavingtechniques.

In addition to the dyes described in the above examples, it is possibleto use indigo, thioindigo, bis-2,1-naphthioindigo, and similar indigoand thioindigo vat colors. Some improvement in dyeingwith: certainanthraquinone vat colors is also obtained upon using the potassiumcompounds but the effect is much less marked than that obtained with theindigo and thioindigo dyes. It is preferred to use an indigo or athioindigo dye in any desired amount. By the use of any of the above orother indigo and thioindigo vat colors, strong shades that are fast towashing and crocking are obtained.

The other ingredients of the dye bath and the conditions of dyeing maybe varied widely. Any of the commercial techniques in respect to thesefactors may be employed herein. Standard commercial dyeing processesemploy 30:1 to 50:1 ratios of bath to structure. These ratiosmay be'usedin this invention. In general, any-of the commercial dyeing apparatusand procedures, such as-purlfication and oxidation and the like, may beused herein. v The oxidation of the vat dyestuff on the structure may becarried out in other ways. Instead of using nitric acid, other agentsfor oxidation may be employed, such as aqueous solutions of sodiumdichromate, hydrogen peroxide, sodium perborate and the like.

.While the temperature of the dye bath may be varied widely, it ispreferred to use temperatures above 85 C. Below these temperatures therate of dyeing is tooslow to be commerciallypractical. This rateincreases rapidly with higher temperatures and in the interest ofeconomy it is preferred to carry out thedyeing at a bath temperaturebetween 95 and 100 C. Temperatures above 100 C. may be employed.Particularly good, results are obtained with highly drawn acrylonitrilepolymer yarns when the process is carried out under pressure attemperatures exceeding 100 C., as shown in Example XI.

The vat dyeing of polyacrylonitrile articles in highly alkaline vatbaths should not be conducted at temperatures of greater-than 130 C. forperiods of time greatly exceeding minutes. Higher temperatures andlonger periods oftime generally lead to loss in physical properties. Atthe lower temperatures there is no adverse effect; at the highertemperatures attack by the alkali is avoided by using very short timeperiods. The dyeing oihighly drawn yarns and .fabrics 8 to'strong shadescan be accomplished satisfactorily at temperatures ranging'from about100- l30 C. and periods ranging from l5-30 minutes.

This invention is of particular value in the dyeing of yarns, filaments,and fabrics and simi-' lar articles of polymers containing at leastacrylonitrile. Polyesters, nylon and cotton articles do not respondappreciably tothe process of this invention. This invention afiords theonly process known hitherto for dyeing acrylonitrile polymers containingat least 85% acrylonitrile to strong shades with vat colors Without theuse of an organic carrier or assistant. Since potassium compounds arecheap and since relatively expensive assistants, such as amines, are notneeded, the process of this invention is more economical than processeswhich involve the use of carriers.

The eifect of obtaining the strong shades in the dyeing of acrylonitrilepolymers described in this invention appears specific to the potassiumion. For example, lithium and sodium ions do not act in a similar mannerand are ineffective.

The vat dyestuffs are characterized by their fastness to light andwashing. Consequently, vat dyeing is an important and popular method forthe coloring of textile articles. The sale of textile materials preparedfrom new synthetic polymers is limited unless these materialscan bereadily colored and made more appealing to the public. An important stepin the development of a new synthetic fiber, therefore, is the provisionof a satisfactory method for dyeing the fiber to strong shades usingavailable commercial equipment. This is a difiicult problem when the newsynthetic fiber is hydrophobic by nature; that is. it has little or notendency to swell in the presence of water. Yarns and fibers preparedfrom synthetic acrylonitrile polymers, which are hydrophobic by nature,are not penetrated and colored readily in commercial dyeing procedures.Dyestuffs which are substantive on other types of polymers are notnecessarily so on acrylonitrile polymers. 'In fact, it cannot bepredicted which class of dyestuifs will most effectively color a newsynthetic yarn. Normally, structures prepared from acrylonitrilepolymers cannot be dyed satisfactorily with any dyes, such as acid,direct, sulfur, dispersed acetate, basic or vat colors. At best onlylight shades are obtained.

This invention provides a convenient and valuable method for dyeingstructures prepared from acrylonitrile polymers. By the novel process ofthis invention difiicultly dyed structures of acrylonitrile polymers canbe dyed eifectively at a rapid rate using standard commercial equipment.By the use of the process of this invention, vat dyeing procedures,which when normally applied to acrylonitrile polymer structures give nocoloring or at most only light shades, are capable of rapidly dyeing thestructures to'level shades of satisfactory strengths. The dye bathexhaust is greatly increased and the colored products produced haveexcessively good resistance to washing and cracking and have good lightfastness.

Furthermore, the physical and chemical properties of the dyed structuresare comparable to the desirable properties of the undyed structures.

Any departure from the above description which conforms to the presentinvention is intended to be included within the scope of the claims.

I claim:

1. Aprocess of dyeing an acrylonitrilepolymer containlngat least 85%acrylonitrilewhichcomprises treating said polymer with an aqueous dyebath having a pH of from about to about 12.5 and containing from 2 to 32equivalent weights of an ionizable potassium compound per 100 parts ofsaid polymer, said equivalent weights being expressed in the same unitsas said parts and in solution a reduced vat dye selected from the groupconsisting of indigo and thioindigo vat dyes.

2. A process for dyeing an acrylonitrile polymer containing at least 85%acrylonitrile which comprises treating said polymer at a temperatureabove 85 C. with an aqueous dye bath having a pH of from about 10 toabout 12.5 and containing from 2 to 32 equivalent weights of anionizable potassium compound per 100 parts of said polymer, saidequivalent weights being expressed in the same units as said parts andin solution a reduced vat dye selected from the group consisting ofindigo and thioindigo vat dyes.

3. A process for dyeing polyacrylonitrile which comprises treating saidpolyacrylonitrile with an aqueous dye bath having a pH of from about 10to about 12.5 and containing from 2 to 32 equivalent weights of anionizable potassium compound per 100 parts of said polymer, saidequivalent weights being expressed in the same units as said parts andin solution a reduced vat dye selected from the group consisting ofindigo and thioindigo vat dyes.

4. A process for dyeing an acrylonitrile/methacrylic acid polymercontaining at least 85% of acrylonitrile which comprises treating saidpolymer with an aqueous dye bath having a pH of from about 10 to about12.5 and containing from 2 to 32 equivalent weights of an ionizablepotasslum compound per 100 part of said polymer, said equivalent weightsbeing expressed in the same units as said parts and in solution areduced vat dye selected from the group consisting of indigo andthioindigo vat dyes.

5. A process for dyeing an acrylonitrile/styrene polymer containing atleast 85% of acrylonitrile which comprises treating said polymer with anaqueous dye bath having a pH of from about 10 to about 12.5 andcontaining from 2 to 32 equivalent weights or an ionizable potassiumcompound per 100 parts of said polymer, said equivalent weights beingexpressed in the same units as said parts and in solution a reduced vatdye selected from the group consisting of indigo and thioindigo vatdyes.

6. A process for dyeing a structure prepared from an acrylonitrilepolymer containing at least 85% acrylonitrile which comprises treatingsaid structure with an aqueous dye bath having a pH of from about 10 toabout 12.5 and containing from 2 to 32 equivalent weights of anionizable potassium compound per 100 parts of said structure, saidequivalent weights being expressed in the same units as said parts andin solution a reduced vat dye selected from the group consisting ofindigo and thioindigo vat dyes.

7. A process in accordance with claim 2 wherein the bath is maintainedunder pressure and at a temperature of about 100 C. to about 130 C.

8. A process in accordance with claim 3 wherein glge bath is heated to atemperature of at least 9. A process for dyeing a structure preparedfrom polyacrylonitrile which comprises treating said polyacrylonitrilewith an aqueous dye bath containing about 2.9 equivalent weights ofpotassium carbonate per 100 parts of said polyacrylonitrile, saidequivalent weights being expressed in the same units as said parts andin solution a reduced vat dye selected from the group consisting ofindigo and thioindigo vat dyes, the said bath having a pH of between 10to 12.5 and being heated under pressure at a temperature of about 130 C.

10. A process in accordance with claim 1 wherein the said bath is heatedto a temperature of from about C. to about 130 C. and the treating ofsaid polymer in said bath is carried out for from about 15 to about 30minutes.

11. A process for dyeing an acrylonitrile polymer containing at least85% acrylonitrile which comprises treating said polymer with an aqueousdye bath having a pH of from about 10 to about 12.5 and containing from2 to 32 equivalent weights of an ionizable potassium compound per partsof said polymer, said equivalent weights being expressed in the sameunits as said parts and in solution in said bath a reduced vat dyeselected from the group consisting of indigo and thioindigo vat dyes andthereafter removing the said polymer from the said bath and oxidizingthe said dye.

12. A process for dyeing an acrylonitrile polymer containing at least85% acrylonitrile which comprises treating said polymer with an aqueousdye bath having a pH of from about 10 to about 12.5 and containingsodium hydroxide and 2 to 32 equivalent weights of an ionizablepotassium compound per 100 parts of said polymer, said equivalentweights being expressed in the same units as said parts and in solutiona reduced vat dye selected from the group consisting of indigo andthioindigo vat dyes and thereafter removing the said polymer from saidbath and oxidizing the said dye.

13. A process in accordance with claim 12 wherein the temperature isfrom about 85 C. to about C.

ESTHER MERYL GRIFFITH.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,868,300 Wylam July 19, 19321,868,987 I Moorhouse July 26, 1932 1,959,980 Henderson May 22, 19342,067,930 Kern Jan. 19, 1937 2,086,831 Tolman July 13, 1937 2,318,133Wentz May 4, 1943 2,371,145 Brubaker Mar. 13, 1945

1. A PROCESS OF DYEING AN ACRYLONITRILE POLYMER CONTAINING AT LEAST 85% ACRYLONITRILE WHICH COMPRISES TREATING SAID POLYMER WITH AN AQUEOUS DYE BATH HAVING A PH OF FROM ABOUT 10 TO ABOUT 12.5 AND CONTAINING FROM 2 TO 32 EQUIVALENT WEIGHTS OF AN IONIZABLE POTASSIUM COMPOUND PER 100 PARTS OF SAID POLYMER, SAID EQUIVALENT WEIGHTS BEING EXPRESSED IN THE SAME UNITS, AS SAID PARTS AND IN SOLUTION A REDUCED VAT DYE SELECTED FROM THE GROUP CONSISTING OF INDIGO AND THIOINDIGO VAT DYES. 